IEEE 802.11b is a popular standard for the implementation of high data rate Wireless LAN (Local Area Network) systems. The standard uses modulation techniques based on a spread spectrum. Complementary Code Keying (CCK) code words are used in these systems to obtain high data rates of 5.5 and 11 Mbps. These code words possess good auto correlation properties, which make them very attractive for wireless transmission. Performance of such code words degrades in the presence of multipath conditions that are inherent to indoor wireless application.
To operate in such multipath conditions, Rake receivers are used. A rake receiver is a class of receiver that receives signals on as many multipaths as possible. The Rake receiver combines the signals from all of these paths to produce one clear signal that is stronger than the individual components. The Rake receiver design is generally simplified by linear transformation of a Rake combiner resulting in a channel-matched filter. The coefficients of a channel-matched filter can be obtained using the channel coefficients estimated at a receiver. Delay spread of a multipath channel is obtained by using the channel coefficients. The delay spread of the multipath channel is used to benchmark such receivers. The performance of such receivers is dependent on channel characteristics.
The complexity of signal processing required at the receiver to compensate for a multipath channel is dependent on channel characteristics. A higher complexity receiver may be required to compensate for a higher delay spread in a multipath channel. Therefore, design tradeoff between hardware complexity and performance in the channels is generally required when implementing such receivers, i.e., the complexity of Rake receiver hardware increases as the delay spread increases in the channels to be compensated.